Efficient Electromagnetic Wave Absorption and Thermal Infrared Stealth in PVTMS@MWCNT Nano-Aerogel via Abundant Nano-Sized Cavities and Attenuation Interfaces

Highlights PVTMS@MWCNT nano-aerogel with nano-pore size and abundant heterogeneous interface was fabricated via radical polymerization, sol–gel transition and CO2 drying. The nano-aerogel shows superior electromagnetic wave absorption property (RLmin = −36.1 dB and cover all Ku-band) and thermal infrared stealth property (ΔT reached 60.7 °C). Layered nano-aerogel/graphene film with high EMI shielding and absorption properties was obtained; Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01218-y.


S1.1 Material Structure
shows the XPS spectral of pristine MWCNT used in this work, it is noted that there are some oxygen groups (-OH or -COOH) on the nanofiller surface.Therefore, hydrogen bonding structure could be formed between MWCNT (with -OH or -COOH) and PVTMS (with -Si-OH).The hydrogen bonding was beneficial for enhancing the molecular absorption effect of PVTMS on MWCNT surface.

Fig. S3
Linear shrinkage and density of PVTMS@MWCNT aerogel with various MWCNT content Fig. S3 shows linear shrinkage and density of PVTMS@MWCNT aerogel with various nanofiller content, it is noted that the degree of aerogel shrinkage decreased with increasing MWCNT content after scCO2 drying.Meanwhile, the density of nanoaerogel also slightly decreased with increasing MWCNTs nanofiller content.As Fig. S5a, b shows, comparing with pristine PVTMS aerogel, PVTMS@MWCNT aerogel shows enhanced tensile strength and stretching ratio at break.As Fig. S5c, d shows, the bending modulus of PVTMS@MWCNT aerogel increased from 2.7 MPa to 4.7 MPa by adding MWCNTs.Besides polarization loss, tunnelling loss should be another important mechanism for PVTMS@MWCNT aerogel EMW absorption.Fig. S8 shows the conductive MWCNT structure in PVTMS aerogel system.At this carbon-PVTMS-carbon model, the contact resistance could be calculated based on the following equation [S1, S2]:

S1.3 EMW Absorption Property of PVTMS@MWCNT Aerogel
where V is electric potential difference (V)，A is tunnel cross-sectional area (m 2 )，J is tunnel current density (A/m 2 )，dmin is carbon nanofiller minimum physical distance, P is Planck constant (N•m•s)，m is electron mass (kg)，λ is barrier height (eV)，e is elementary charge (C).
It is noted that carbon nanofiller contact resistance positively correlated with physical distance (dmin).Therefore, the contact resistance will decrease with decreasing nanofiller distance.And when the dmin is at a suitable range, electrons can move between adjacent MWCNTs with high loss.At this situation, the EMW energy will be transferred into joule heat via tunnelling loss.Fig. S10 shows the IR stealth property of PVTMS@MWCNT aerogel with various MWCNT content, it is noted that the IR stealth property could be maintained with increasing MWCNT nanofiller content.This could be ascribed to the nanopore size generated in the nano-aerogel system greatly decreasing the thermal conduction.It is well known that all matter with a temperature above absolute zero constantly emits thermal radiation (infrared radiation).Infrared waves cover the range of 0.76 to 1000 µm and can be sub-divided into five parts: Near-infrared (0.76-1.5 µm), Short-wave infrared (1.5-3 µm), Medium-wave infrared (3-8 µm), Long-wave infrared (8-15 µm) and Far-infrared (15-1000 µm).It is worth noting that the Earth's atmosphere absorbs most of the infrared and is only relatively transparent to electromagnetic waves in the 3-5 and 8-14 µm range.Therefore, infrared emissivity in the 3-5 and 8-14 µm range are important for IR stealth application [S3].
As Fig. S11a, b shows, infrared emissivity results for PVTMS@MWCNT-1.7 vol% at 3-5 and 8-14 µm range were tested.It is noted that our PVTMS@MWCNT samples shows high infrared emissivity (0.95 at 3-5 µm and 0.94 at 8-14 µm).The proposed reason for the high infrared emissivity could be ascribed to the added MWCNT, which could act as black body, hence to absorb and emit IR signal.As Fig. 4c shows, the PVTMS@MWCNT nano aerogel in this work could greatly suppress the heat transfer from hot target to the upper surface.For PVTMS@MWCNT nano aerogel, it could be divided into two parts: (1) ultra-low thermal conduction part (PVTMS nano-aerogel); (2) high thermal conductivity part (MWCNTs); As Fig. 2d shows, PVTMS nano-aerogel structure shows even lower thermal conductivity than air (26 mW•m -1 K -1 ).Therefore, as Fig. S12c shows, PVTMS with low thermal conductivity could greatly enhance the overall thermal resistance of PVTMS@MWCNT system [S4, S5].

Fig. S5
Fig. S5 Digital photos for stretching mechanical property test (a), stretching stressstrain curves of samples (b), Digital photos for bending mechanical property test (a), bending stress-strain curves of samples (b)